It is known to use directional drilling apparatus to form substantially horizontal drill holes in the ground for installing electrical or telephone cables, gas or water pipes, or the like. The directional drilling equipment usually comprises a percussion drill bit operable through a drill string and includes a steering device so that the drill bit can be steered in a desired substantially horizontal direction below and along the route of roads and streets, and under river beds, roads and the like. Such directional drilling apparatus is well known and is disclosed, for example, in WO 97/49889, U.S. Pat. No. 6,705,415, and US 2004/0188142A.
In a typical directional drilling system, a drill string incorporating a percussion hammer, to apply axial impact forces to the drill bit, is utilised to drill an initial pilot hole of a small diameter (for example 133 mm) than the diameter of the ultimate passage desired. The ultimate drill passage may have a diameter of anything from 200 mm to 760 mm depending upon the size of the cables, pipes or conduits to be inserted in the drilled passageway.
With the known horizontal drilling method, when the percussion drill bit and hammer breaks through the surface after having drilled the pilot hole, the percussion hammer system is removed and a reamer bit is fixed to the protruding drill rod. Using the drill string, the reamer bit is then pulled back against the face of the drilled pilot hole. By means of a rotary force only (i.e. without percussion) the hole is then reamed by the reamer bit to the required larger diameter. Because there are no percussion forces involved the penetration rate of the reamer is very slow, particularly in hard rock formations. Back-Reaming tools are disclosed, for example, in US 2002/0108785A,
Another known method of enlarging the pilot hole is to remove the drill string including the percussion drill hammer from the pilot hole and then using another hammer system with a larger bit to ream the hole to size. For example, U.S. Pat. No. 4,249,620 discloses a method of boring holes which includes forming an open-ended pilot bore by means of a first self-propelled displacement hammer of smaller diameter. Next a cable is introduced through the pilot bore and then the diameter of the pilot bore is increased to form the hole by means of a second displacement hammer of greater diameter. The second displacement hammer is guided through the pilot bore by pulling it with the cable, which for this purpose is attached to the leading end of the second hammer. The disadvantage of this system is that because the initial drill string has been removed there is a danger that the larger hammer system will deviate from the course of the pilot hole. For example, it may drift to one or other side of the axis of the pilot hole. Also there can be a risk of the pilot hole collapsing in broken conditions.
It is known to use similar methods to enlarge pilot holes drilled by raise boring apparatus in the drilling of vertical elevator shafts and the like. The pilot hole is drilled vertically downwards, and then using the pilot hole as a guide, a drill head is pulled upwardly to enlarge the hole. An example of such a drill head is disclosed in US 2004/0188142A. This drill head utilises at least two, and preferably three impact hammers.
EP 0 507 610 A (Rear) discloses an uphole hammer comprising a substantially tubular housing which is closed at one end and supports a substantially centrally located fluid supply tube at said one end. The fluid supply tube extends axially through the housing and is connected at its other end to drill a string, and receives fluid being directed to the hammer by the drill string. The housing supports a drill bit at its other end, which is slidably received about the fluid supply tube. A piston also is slidably supported about the fluid supply tube in the housing for reciprocation between the drill bit and the end of the housing. Fluid porting means is provided to alternately admit fluid to the spaces defined between each end of the piston and the respective ends of the housing to effect reciprocation of the piston between a first position at which it impacts on the drill bit and a second position at which it lies in the vicinity of the end of the housing.
The uphole hammer disclosed in EP 0 507 610 A suffers from a number of disadvantages. It discloses a centrally located fluid supply tube (13) which extends axially through the hammer housing (11). The tube serves to transmit torque and to regulate the piston cycle. Because of the complicated machined shapes of the tube and the provision of ports 30a and 30b cut through the wall of the tube, the structure of the tube is significantly weakened for the purpose of torque transmission. The clearance between the piston and the tubes must be sufficiently small to provide the sealing necessary to operate the piston cycle. The small clearance in conjunction with the torque transmission, and weakening of the wall structure, places excessive stress and bending on the tube. For example because of the presence of ports 30a and 30b, and the requirement for a tight running clearance with the piston, it is likely that the tube will distort under torque. Avoiding the clearance problem may necessitate that the clearance between the piston and the tube be increased, thus reducing efficiency.
Also, in EP 0 507 610 A, the drill bit (18) is retained in the drivel-sub bit support (17) by a bit retaining ring (19), which results in a relatively weak bit design. Furthermore, in EP 0 507 610 A, the back end of the hammer comprises an end plate (12) fixed to the tubular housing (11) by studs (15) and to one end of the fluid supply tube (13) by a second set of studs (16). There is a high risk that in operation the variation of the system would cause studs to loosen which would result in very serious damage to the hammer.